Slow Onset of Activation and Delay of Inactivation in Transient Current of Nitella axilliformis

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Abstract
Single membrane samples of Nitella axilliformis, in which major parts of vacuoles were removed, were prepared by centrifugation and ligation with threads. Voltage clamp experiments were made with the samples, proving that the transient current occurs only after a initial delay as was observed in Chara corallina by Beilby and Coster [(1979) Aust. J. Plant Physiol. 6 : 337]. The membrane potential measured at a corner of the sample exhibited practically the same time course as the membrane potential at the center which changed stepwise, indicating that the delay is not related to the propagation of the potential change. The delay did not change sensitively to ionic strength of the external solution, suggesting that it is not caused by low electric conductance around the sample but is related to the gating mechanism of Cl channel. Various models were examined to explain the time course of the transient current. The best agreement was obtained by introducing a delay δn in inactivation with the expression of IC1=gC1m8h(VM−VC1), where IC1 stands for the transient current and VM is the clamp potential, and gC1 = 15mS·cm−2, VC1= − 31.5 mV. The delay δn decreases similarly to τn with increasing VM, suggesting that inactivation starts after activation proceeds.